The relative abundance of stable isotopes of elements such as carbon, hydrogen, oxygen and nitrogen varies slightly but significantly in various environments. Determination of isotope ratios is a frequently used research tool, for example in forensic, anthropological, biochemical and environmental research, as well as in drug and food industries and for the determination of doping amongst athletes.
Isotope ratio analysis is a methodology for determining the relative abundance of isotopes, for example in gaseous samples containing CO2. For example, isotope ratio analysis can be used to determine the isotope ratios of carbon and oxygen, e.g. 13C/12C and/or 18O/16O. Isotope ratio analysis is most commonly performed by optical spectrometry and mass spectrometry.
Gas inlet systems for isotope ratio analysis are known in the art, especially for mass spectrometers. A general review of isotope ratio mass spectrometry and gas inlet systems is provided by Brenna et al., Mass Spectrometry Reviews, 1997, 16:227-258.
Determination of the isotope ratio of samples usually requires a comparative measurement of the isotope ratio of a sample gas and one or more reference gases with a known isotope ratio. Gas inlet systems for isotope ratio analysis therefore usually comprise an analyte gas inlet, for providing sample and/or reference gas into the analyzer, and a carrier gas inlet, for providing a carrier gas for facilitating transfer of analyte, and that usually does not contain the gas to be determined by the analyzer.
In isotope ratio optical (usually infrared) spectrometry (IROS), photo absorption by CO2 or H2O is measured and isotope composition determined from the resulting spectra. This methodology has advantages over mass spectrometry, for example due to ease of use, cost and portability. Further, IROS allows direct analysis of H2O, whereas isotope ratio mass spectrometry requires conversion of H2O to e.g. H2 or CO2, or equilibration with CO2, prior to analysis.
For IROS, however, it is especially critical that the pressure remains constant during the measurement. This is because the isotope ratios are determined by a fit of the adsorption spectrum; the peak shape and width of the adsorption bands depends on the pressure. This is as more important as the fit parameters are optimized for a defined pressure.
Optical cells usually measure samples in a continuous sample flow, or by stop-flow. In a continuous flow, a sample gas is pumped through the optical cell at a constant flow. Pressure is usually maintained by a regulating pump, or by means of a valve at the outlet from the cell. Input into the cell is typically either uncontrolled, i.e. open to atmosphere, or it is controlled by a flow controller. When samples are measured by stopped-flow, the optical cell is flushed with sample, after which the flow is stopped by use of valves at the inlet and outlet of the cell during the measurement time.
A distinct disadvantage of the continuous flow method is that it consumes a large amount of sample, since there is a constant flow of sample through the optical cell during the measurement time. Reducing the flow into the optical cell can reduce sample consumption, but at the same time the response time of the system, i.e. the filling and flush response time, is increased. Stopped-flow however, while reducing the amount of sample that is required, suffers from several drawbacks, such as leaks in the cell, dead volumes and surface effects, that make this method very unstable. For stopped flow it is especially critical that the pressure during the measurement has to be at a defined value and has to be kept constant. Even the switching of a valve normally results in pressure changes, which cannot be compensated in stopped flow technique.
WO 2014/170179 discloses a gas inlet system for introducing gas into an isotope ratio analyzer. The system includes a reference system that comprises supplies of reference and carrier gas that are connected at a mixing junction where they combine. The gases are mixed within a mixing zone and further transported to the isotope ratio analyzer via an exit line, that also comprises an opening.
Sturm et al. (Atmos Meas Tech, 2010, 3:67-77) describe a water vapor isotope analyzer (WVIA, Los Gatos Research Inc), in which the flow of gas can be changed by changing the speed of the pump.
Johnson et al. (Rapid Commun Mass Spectrom, 2011, 25:608-616) discloses a system for an isotope ratio laser spectrometer, that includes a needle valve in front of the analyzer for regulating flow rate.
U.S. Pat. No. 7,810,376 discloses a system for controlling gas flow such that the partial pressure of an analyte gas is held constant. The system includes a controller that controls gas flow in a sample chamber by mixing an inert gas into the chamber so as to keep the partial pressure of the analyte constant.
It would be desirable to provide a gas inlet system for isotope ratio analyzers that provides for rapid analysis of samples while simultaneously minimizing sample loss.
The present invention has been made against this background, to provide a solution to the aforementioned problems and at the same time provide additional advantages as described in the following.